The present invention relates to a power supply, in particular a switching power supply for protecting electric elements from overcurrent caused by late switching operation of a switching element involved in the power supply.
A prior art DC-DC converter is one of switching power supplies that comprise a transformer having primary and secondary windings; a switching transistor connected in series to the primary winding of the transformer and a DC power source; a current detection resistor connected in series to the switching element for detecting electric current through the switching element; an output smoothing circuit connected to the secondary winding of the transformer; and a control circuit for producing drive signals of controlled pulse duration to a control terminal of the switching transistor. A feed terminal of the control circuit is connected to the DC power source through a starting resistor and to one end of a drive winding of the transformer through a diode. The control circuit has a pulse generator or oscillator for generating periodic pulses that are supplied to various circuits in the control circuit.
If the switching transistor is operated with increased or higher switching frequency, a considerable time delay occurs between periodic pulses from the oscillator and OFF switching of the switching transistor so that the switching transistor cannot timely be turned ON and OFF in exact synchronization with output pulses from the oscillator. With higher switching frequency of the switching transistor, OFF switching timing of the switching transistor is more delayed for output pulses from the oscillator so that a primary current flows through the switching transistor during the delayed period of time until OFF switching of the switching transistor to thereby cause overcurrent to flow through the secondary winding, and remarkable surge voltages occur on the secondary winding and drive winding of the transformer while the control circuit cannot be turned OFF due to increased voltage produced on the drive winding. In this way, remarkably increased peak current flows through the primary winding, and it may cause damage to the switching transistor and breakdown of other electric elements, however, the switching transistor cannot have its performance capable of switching with shorter ON width.
An object of the present invention is to provide a switching power supply with overcurrent protection capable of effectively controlling excess current resulted from late OFF switching of a switching element to prevent breakdown of electric elements.
The switching power supply according to the present invention comprises a transformer (2) having primary and secondary windings (3, 4); a main switching element (5) connected in series to a DC power source (1) and the primary winding (3) of the transformer (2); an output smoothing circuit (6) connected to the secondary winding (4) of the transformer (2); and a control circuit (7) for producing drive signals to a control terminal of the main switching element (5) to turn the main switching element (6) ON and OFF and take out DC output from the output smoothing circuit (6); current detection means (8) for detecting electric current through the main switching element (5); storing means (100) for accumulating outputs from the current detection means (8); and voltage control means (200) for reducing source voltage applied to a feed terminal of the control circuit (7) below an operation voltage when output form the storing means (100) is over a predetermined value. The current detection means (8) detects the current through the main switching element (5) and produces detected outputs that are accumulated in the storing means (100). When excess current occurs due to late ON to OFF switching of the main switching element (5), the storing means (100) accumulates the excess current. When the accumulated voltage in the storing means (100) exceeds a predetermined level, the voltage control means (200) is operated to reduce the source voltage for the control circuit (7) below the operative voltage or to the OFF level to stop operation of the control circuit (7) and to thereby restrain excess current during delayed time until OFF switching of the main switching element (5). Deactivation of the control circuit (7) protects the main switching element (5) and output smoothing circuit (6) against excess current, and reducing electric stress thereon to prevent breakdown of these elements.